Life cycle phases: Literature review and new classification proposal for application in healthcare

ABSTRACT Introduction: Although uncommon in dentistry, the concept of the life cycle holds great importance for dental professionals in identifying crucial intervention opportunities and determining the optimal timing for treatments and procedures. Objective: To carry out a review of the literature on life cycle classifications and their distinct phases, evaluating their applicability in healthcare. Methods: A literature review was performed, searching for articles in PubMed, SciELO, National Health Library (BvB), and Google Scholar databases, as well as relevant books. The keywords “life cycle,” “life stages,” “human development,” “age groups,” and “biological age” were used. Relevant articles were selected by analyzing their titles and abstracts, and read in full to confirm their inclusion in the research. Results: Nine distinct life cycle classifications were found, each with unique criteria. Conclusion: Based on the comprehensive literature review, a novel classification was proposed (The 10-phase Life Cycle), which encompasses dental, growth, physiological aging, sociocultural, and behavioral characteristics, aiming to enhance communication among healthcare professionals, particularly those engaged in the growth, development, and aging processes of human beings.


INTRODUCTION
The increase in life expectancy has led to increased longitudinal studies and studies related to the life course, including the changes that occur during the life cycle.A person's life cycle is commonly divided into sequential phases, which follow their age, from birth to death.These phases, which have a predictable duration, can also be demarcated from a perspective of biological events (puberty, menarche, reproduction, and menopause) and from a social events perspective (marriage, parenting, and retirement). 1us, the life cycle phases can be measured from developmental markers, chronologically defined, considering the amount of time elapsed from conception or birth to the end of life. 2 Chronological age can be a reference axis for persons, and it has the advantage of presenting similar conditions according to sex, race, or social status. 3 However, because of social, historical, and cultural changes, the delimitation of life phases has been changing, making the chronological criterion valid but insufficient to characterize the life cycle, 4 since persons of the same age may present different biological conditions. 3In Western societies, age groups chronologically organized, based on a chronological system, are a crucial tool even for defining and attributing status -such as legal adulthood status-, insertion in the labor market and the right to retirement. 5However, the use of chronological age, without detailing the criteria used, 6 requires more complete classifications.
The use of well-defined phases of the life cycle in the health area is essential for professionals to recognize the windows of opportunity for intervention and the most appropriate time to conduct treatments.Identifying the characteristics of each phase of the life cycle becomes essential for health professionals to act appropriately in growth, development, and aging.This knowledge makes it possible to understand the normal patterns, conflicts, and expected disorders, 7 besides facilitating communication between professionals.However, establishing a standard for human life is a challenge because this reflects several aspects, including cultural and social ones. 8e present study comprehensively reviewed the existing literature on life cycle classifications, considering their distinct phases.Based on the findings, a novel life cycle classification was proposed, based on dental, growth, physiological aging, sociocultural, and behavioral characteristics that facilitate communication between health professionals, particularly those who act in human growth, development, and aging.

METHODS
A review of the literature was carried out, searching for articles in the PubMed, SciELO, National Health Library (BvB), and Google Scholar databases, from 1972 to 2020.The Portuguese keywords "ciclo vital", "estágios do ciclo de vida", "desenvolvimento humano", "faixas etárias", "grupos etários", "idade biológica", and their equivalents in English ("life cycle," "life cycle stages", "human development", "age groups") were used.The terms were associated with each other by the Portuguese operators "E" and "OU," and their English counterparts, "AND" and "OR."From the search results, the pertinent publications were selected through the analysis of their titles and abstracts, both in Portuguese and in English.The chosen articles were thoroughly read to confirm whether they should be included or excluded from the research.The criteria for inclusion encompassed articles and book chapters that were published with full text, in Portuguese or English, presenting characterizations of the life cycle phases, with no limitation on publication date (Fig 1).In order to identify other relevant publications, the reference list of the included studies was manually checked for relevant or missing studies using the keywords previously mentioned.Studies that met all inclusion criteria were selected.When there was no mutual agreement between the two authors responsible for selecting the articles, a third author was involved to make a final decision.The literature review was divided according to the different classifications found, and the results were presented and discussed.

RESULTS
Based on the bibliographic material consulted, 9 classifications were registered for the life cycle, which will be described according to the number of phases, presenting their criteria and nomenclatures.From these nine classifications found, graphs with colors were created, representing the life cycle phases, as described by each author.According to Bogin and Smith, 9 after birth, the life cycle is described in five phases: infancy, childhood, juvenile, adolescence, and adulthood (Fig 3).These phases are characterized as follows: » Infancy: starts with birth, or, more accurately, when the person first receives nutrition via lactation, be it exclusive or non-exclusive breastfeeding, and ends with weaning.In pre-industrialized societies, this occurs from 0 to 36 months.
» Childhood: the period after weaning, in which the person still depends on someone for food that must be prepared considering the dentition and digestive tract immaturity, for protecting the person against diseases.The important milestones for the completion of the stage are the eruption of the first permanent molar and complete growth of the brain, which correspond to the possibility of nutrition like that of an adult, because of the masticatory capacity and increased cognitive capacity.The events that determine the end of childhood occur around 6.5 to 7 years of age.
» Juvenile: phase preceding puberty, in which persons are no longer so dependent on their parents for food 10 and have greater physical and cognitive capacity. 11,12For girls, this period ends around the age of 10, while for boys it occurs 2 years later.
» Adolescence: starts with puberty, when signs of sexual maturation are manifested, such as the appearance of pubic hair.In this phase, growth is accelerated, characterizing the "growth spurt".The end of this phase is related to the achievement of stature and complete sexual maturity.
For women, it occurs, on average, at 19 years of age, and in men, around 21 to 25 years.
» Adulthood: bone apposition ends, and resorption begins around the fifth decade of life.» Third age: 60 years or more, corresponds to the retirement phase, understanding of the proximity to the end-of-life, reduction of income, strength, and health.» Infancy (under 1-year old).
The United Nations addresses different definitions, with a greater or lesser range of ages, for use according to the required purpose, subdividing them into the following phases: preschool children   » Third age: 65 years and older, the last phase, being described as when retirement and a decrease in physical capacity and health commonly occur.-Senility: variable duration phase, according to somatic and functional reserve levels.
-Death: reduction of tissue and organ performance below the minimum support necessary for the maintenance of life.

DISCUSSION
The literature presents different classifications of the life cycle phases, which focus on chronology and social or biological issues.
However, these classifications are not standardized, making communication between researchers and scholars difficult.
It was observed in the researched literature that among the classifications that were found, five described the life cycle in at least seven phases, concentrating more than half of the cycles before the adult phase.The phases related to old age were scarcely described.To classify the phases of the vital human cycle, besides biological age, morphological (measurements of height and weight compared to normative indices), skeletal (use of radiographs of the carpal bones of the hand and wrist to determine skeletal age), dental (sequence of eruption and tooth formation), and sexual aspects (analysis of primary and secondary sexual characteristics) should be considered.
One could also include emotional (socialization) and mental aspects (cognitive/learning potential), self-perception (the person's assessment of their ability), or perceptive age (personal assessment of development rate). 15e classifications by Gonçalves However, the authors did not mention how to evaluate the growth stage when citing the growth spurt, both to demarcate the beginning and the end of the adolescence phase.In adulthood, they describe the end of bone apposition and the beginning of resorption around the fifth decade of life.
Based on chronological age, the United Nations 14  vertebral maturation analysis (CS3 and CS4). 20The ascending curve of the pubertal growth spurt comprises the stages FD=, FP=, FM=, G1, Psi, R=, FDcap, S, and G2, 21,22 described in Table 2 and shown in Figure 12.The peak of the pubertal growth spurt generally begins at 10 years and 7 months in females and at around 12 years and 7 months in males. 20,23This peak of pubertal growth spurt can be confirmed by the analysis of the carpal radiography of an individual between the phases FPcap (left-hand capping of the proximal phalanges of the middle finger) and FMcap (left-hand capping of the median phalanges of the middle finger).Dental aspects are also important markers of the life cycle, and the phase of dentition and its characteristics should be evaluated.The "Ugly Duckling Phase" in mixed dentition is a physiological stage in the development of human dentition that occurs between 8 and 10 years of age, and it is characterized by the distal inclination of the crowns and the presence of diastema between the upper incisors, overbite, and exaggerated overjet. 24This physiological example of a characteristic of dentition development exposes the importance of defining a more appropriate life cycle classification for the health area, especially for professionals who work with growth and development, because, at this age, at least eight nomenclatures could apply to this phase: youth, 9,14,18 school age, 17 adolescence, 9,13,15,[17][18][19] third childhood, 8,16 childhood, 15 second childhood, 13 preadolescence, 8,13 and puberty. 18In this way, the attempt to classify a research sample over the distinct phases of the life cycle so far proposed could generate bias, since several nomenclatures can be applied to the same phase, which is not always similar.This is the case of research that evaluates the presence of malocclusions and their relationship with the forms of breastfeeding and children's habits, which would involve children between 2 and 4 years of age; these phases are classified in diverse ways: baby, first childhood, or middle childhood. 9,14,25us, in the present article, the authors suggest a classification for the life cycle, divided into 10 phases (Fig 11), based on dental, growth, physiological, socio-cultural, and behavioral aging characteristics.Chronological age must be considered.
However, the classification should be based on the predominant characteristics in each phase, and not only on the age group.» Breastfeeding. 26Predental period, known as the period of gingival rollers, is characterized by the absence of teeth. 27Physiological distocclusion: The mandible is retruded in relation to the maxilla, with a discrepancy of 3 to 5 mm. 28Normally, from birth to 29 days.

BABY
» Breastfeeding and, beginning at six months, solid food introduction. 18Period of the deciduous dentition, which begins with the eruption of the first deciduous tooth. 2 » Physiological growth of the mandible, enhanced by breastfeeding, promotes necessary muscular stimulation for development. Normally, from 30 days to 23 months.

PRESCHOOL CHILDHOOD
» The complete deciduous dentition marks the beginning of this life cycle's phase.
» Development of cognitive skills. 9 » The body assumes proportions like that of an adult. 16Intersphenoidal synchondrosis fusion. 29,30Normally, from 2 to 5 years old.» Ugly duckling phase. 24Logical thinking development, language, and memory improvement. 16Fusion of spheno-ethmoidal synchondrosis. 29,30Usually, the individual is between the pre-peak phases of the pubertal growth spurt (between FD = and FM =), or in the beginning of the pubertal growth spurt (G1) or in the ascending phase of the pubertal growth spurt (Psi), or at the peak of the pubertal growth spurt (between FPcap and FMcap), determined by analyzing the middle finger of the left hand through carpal radiography 21,22 (Table 1 and Fig 12).
» Normally, from 6 to 12 years old.Marks the end of maxillary growth; body, structural, and mandibular growth may extend for up to 1 or 2 years after total radius epiphyseal union (growth may occur as long as there is a cartilaginous radiolucent line between the epiphysis and diaphysis of the radius)

ADOLESCENCE
» Transition from the late phase of the mixed dentition to permanent dentition: the eruption of permanent second molars and/or permanent canines.
» Post-menarche phase for girls (estimated growth of approximately five years more). 31Vertebral maturation analysis: stages CS3 and CS4. 20Through the analysis of the carpal radiograph, it can be determined whether the patient is at the ascending phase of the pubertal growth peak curve (between R= and G2), at the peak of the pubertal growth spurt (between FPcap and FMcap), at the beginning of the descending curve (Rcap and M-Fdui); at the descending curve of pubertal growth (between Fpui and Riu); or at the end of growth (Rut) 21,22 (Table 1 and Fig 12).
» Search for identity and maturity of the reproductive system. 16Normally, from 13 to 18 years.

YOUNG ADULT
» Characterized by permanent dentures and the presence of third molars.
» Stability of personality traits. 16Physical and cognitive peak. 8,19Maximum performance of adult life.» Maturation of the cervical vertebrae shows that the patient is no longer growing; 20 the carpal radiograph shows stages of the downward curve, or the end of the pubertal growth spurt (stages FPut, FMut, Riu or Rut), 21,22 as shown in Table 1 and Figure 12.
» Beginning of synostosis of the coronal and sagittal sutures, 32 and fusion of the spheno-occipital synchondrosis. 33Normally, from 19 to 24 years.

ADULT
» Complete permanent denture.
[22] » Increase in the number of marital relationships and paternity. 16Synostosis of the lambdoid suture. 32Normally, from 25 to 44 years.

MIDDLE AGE
» Characterized by mature permanent dentition, dental wear, arch length, and perimeter reduction, and decrease in the overbite, besides an increase in lower anterior crowding. 34First signs of physical degeneration, characterized by menopause for women, 35 and reduced sperm quality for men. 18Affected reproductive capacity. 8Normally, from 45 to 64 years old.» Teeth in more yellowish, brown, or gray tones; teeth with greater susceptibility to erosion, abrasion, plaque accumulation, and periodontal disease. 36Adaptation of the stomatognathic system in the face of morpho-functional conditions, which become slower, uncoordinated, and adapted to the structural damage that occurs over the years. 37Process of bone resorption in the maxilla and mandible, increasing its porosity; jawbone thickness reduction and mandibular angle decrease. 36Decreased proximal femur mineral density, increasing the risk of hip fractures. 38Increased physical and cognitive frailty. 8In general, from 65 to 79 years old.

OLDER ADULTS + (80 YEARS OR OLDER)
» Partial or total absence of teeth associated with the use of prostheses.
» Partial or complete obliteration of the volume of the canals and pulp chamber. 39Reduced dry mouth perception and difficulty for eating; the higher need for prosthesis. 36,37Higher frequency of chronic pain. 40 distinct phases of the life cycle proposed in this study can be seen in Table 2, distributed according to the characteristics presented.
This article was written in response to the need for a life cycle classification that provides clear and well-defined boundaries for its phases, specifically for use in scientific research within the healthcare field.Moreover, due to the growing life expectancy, some existing classifications grouped individuals into vastly different stages of maturation, cognition, and requirements.As a result, the proposed life cycle classification seeks to expand the viewpoint and promote contemplation on this matter.

CONCLUSION
In this study, we conducted a comprehensive review of existing studies on life cycle classifications, focusing on their distinct phases.Based on the present analysis, the authors propose a classification that considers dental, growth, physiological aging, sociocultural, and behavioral characteristics.The primary aim of this classification is to enhance communication among healthcare professionals, particularly those involved in human growth, development, and aging.By providing a framework that encompasses these various aspects, the authors believe it will facilitate a more comprehensive understanding and effective collaboration in this field.

Menezes 5 Dental
LM, Neves BM, Motta BB, Azeredo F, Quintão CCA -Life cycle phases: Literature review and new classification proposal for application in healthcare

Figure 1 : 9 )
Figure 1: Flowchart of collection of bibliographic material to carry out the research.

LIFE CYCLE CLASSIFICATIONS: THE 3 -
PHASE LIFE CYCLE (CAMARANO, 2006)According to Camarano, 1 the life cycle is divided into three phases, the first being a junction between childhood and adolescence, the second represented by adulthood, and the third corresponding to old age (Fig2).The life cycle organized in this configuration concerns the person's position in the labor market and the family constitution.1

Figure 3 :classification proposal for application in healthcare 10 Dental
Figure 3: Graph with life cycle in 5 phases, based on Bogin and Smith's classification (1996).9

(from 1
to 4 years); school-age children (from 5 to 14 years); adolescence (phase between childhood and adulthood that comprises the ages from 10 to 19 years, with rapid physical, cognitive and psychosocial growth); third age (persons aged 64 years or older).

Figure 5 :classification proposal for application in healthcare 13 DentalTHE 7 -»
Figure 5: Graph with life cycle in 6 phases, based on the United Nations classification (1982).14

Figure 6 :classification proposal for application in healthcare 15 Dental»»THE 8 -»»»»
Figure 6: Graph with life cycle in 7 phases, based on the classification by Gallahue et al. 15 (2013), with its subdivisions.

Figure 7 :
Figure 7: Graph with life cycle in 8 phases, based on the classification by Papalia et al. 16 (2006).

Figure 8 :
Figure 8: Graph with life cycle in 8 phases, based on the classification by Erik Erikson 17 (2013).
or more (Older adults) Menezes LM, Neves BM, Motta BB, Azeredo F, Quintão CCA -Life cycle phases: Literature review and new classification proposal for application in healthcare Dental Press J Orthod.2023;28(5):e23spe5 » Young adult (19 to 40 years): a period in which the main point is the establishment of mature and lasting relationships.» Adult (41 to 65 years): phase of personal affirmation at work and in the family.» Older adults (65 years and beyond): last phase, in which the lived experiences are reassessed.THE 8-PHASE LIFE CYCLE (BOGIN, 2020) Barry Bogin 18 classified the life cycle into 8 phases (Fig 9) that are described according to age group, length, biological, sociocultural, and cognitive signs: » Neonatal stage (birth to 28 days): characterized by postnatal adaptation, ability to distinguish breast milk odor and reflexes guided by sound and light.

Figure 9 :
Figure 9: Graph with life cycle in 8 phases, based on the classification by Bogin 18 (2020).
Neves BM, Motta BB, Azeredo F, Quintão CCA -Life cycle phases: Literature review and new classification proposal for application in healthcare 19 Dental Press J Orthod.2023;28(5):e23spe5 » Breastfed infant (from 2 months to the end of lactation, occurring around 30-36 months): with food coming totally or partially from milk, whether maternal or industrialized, associated with rapid cognitive, behavioral, and motor development.-Early nursling (2 to 12 months): is characterized by the introduction of other foods from 6 months onwards, the eruption of some deciduous teeth, and completion of the phase marked by bipedal walking.-Late nursling (12 to 30-36 months): development of verbal skills and finalized with the complete eruption of deciduous teeth, besides weaning.» Childhood (3 to 6.9 years): period with moderate growth rate, well-developed bipedal walking, improvement in communication, independence, and self-care.This phase ends with the eruption of the first permanent molars and incisors and with adrenarche.» Youth or prepuberty (7 to 9 years): characterized by a low growth rate, legal school age in most societies, and ability to feed independently.» Puberty (9 to 10 years): neuroendocrine changes in the reproductive system, appearance of secondary sexual characteristics (darkening and increase in pubic/armpit hair, breast development in girls, and genital changes in boys), intensification of friendships and social activities.

Menezes
LM, Neves BM, Motta BB, Azeredo F, Quintão CCA -Life cycle phases: Literature review and new classification proposal for application in healthcare 20 Dental Press J Orthod.2023;28(5):e23spe5 » Adolescence (11 to 22 years): There is a growth spurt and intensification of social, economic, and sexual activities.The duration of this phase is different for girls (11 to 18 years) compared to boys (11 to 22 years).-Prefertility (11 to 13 years old): subcategory ending with menarche for girls and spermarche for boys.-Fertility (13 to 22 years): This phase is characterized by changes in lipid and muscle tissues related to sex, and increased physical and cognitive levels for work.This phase ends with the eruption of the third molars, the end of the fusion of epiphyses of long bones, and the maximum height reached.» Adult life (from 18-22 years to immediately before death) -Prime: a phase that corresponds to the maximum performance of adult life, ending at the age of 35.-Gradual decline (35 to 50 years): It is observed the clinical detection of the first signs of physical degeneration, ending with menopause for women and, for men, reduced sperm quality.-Transition or degenerative age (50 years to senility): decreased cognitive and functional abilities.

THE 12 -
PHASE LIFE CYCLE (GONÇALVES, 2016)Gonçalves,8 based on a study by Papalia, Olds and Feldman,19 presented the 12 phases of the life cycle (Fig 10), with reference to age group, communication characteristics, development, interpersonal relationships, among other aspects evaluated in Psychology: » Conception: begins with fertilization of the ovum.» Prenatal (between 0 and 14 days up to 9 months): zygote (cell multiplication), embryo (beginning of organ formation) and fetus (body growth and improvement) stages.» First childhood (0 to 3 years): in this phase the child develops two important skills: speech and locomotion.» Second childhood (3 to 6 years): discovery of sexuality and construction of gender identity and frequent use of imagination.

classification proposal for application in healthcare 22 Dental»»»classification proposal for application in healthcare 23 Dental
years (Middle age) End of the life cycle (Death) Menezes LM, Neves BM, Motta BB, Azeredo F, Quintão CCA -Life cycle phases: Literature review and new Third childhood (6 to 11 years old): slower physical development occurs, compared to the two previous stages, besides the beginning of school learning and friendships building.» Preadolescence (11 to 14 years): intense body changes occur abruptly.» Adolescence (14 to 17 years): is marked by the relationships building, in the form of friendship "groups".» Youth (17 to 20 years): transition to adult life, with increased social pressures and, therefore, with increased depression rates.Young adult (20 to 40 years): the physical and cognitive peak occurs, and is characterized by the themes of maternity/ paternity, profession, and marriage.Middle age (40 to 60 years): beginning of decline in physical capacity, procreation, children leaving home, preparation for retirement.» Older adults or old age (60 years and older): increased physical and cognitive frailty, retirement, and free time.» Death: represents the end of the life cycle.Menezes LM, Neves BM, Motta BB, Azeredo F, Quintão CCA -Life cycle phases: Literature review and new

8 classification proposal for application in healthcare 24 Dental
and Papalia et al. 16 used chronological, biological, and behavioral criteria.Camarano's 3-phase life cycle classification 1 used criteria related to socio-cultural factors, showing that entering the labor market occurred during or after the person reached adulthood, without using the chronological Menezes LM, Neves BM, Motta BB, Azeredo F, Quintão CCA -Life cycle phases: Literature review and new Press J Orthod.2023;28(5):e23spe5 time to define these phases.The classification proposed by Bogin and Smith 9 in 5 phases presented factors beyond establishing age groups, such as developmental, sexual, and socio-cultural characteristics.The authors correlated dental development in the youth phase with the end of youth.They also related the youth and adolescence phases to the different ages of males and females.

Figure 11 :
Figure 11: Graph with life cycle in 10 phases, based on the proposed classification.

classification proposal for application in healthcare 27 Dental
years (Older adults) Menezes LM, Neves BM, Motta BB, Azeredo F, Quintão CCA -Life cycle phases: Literature review and new

Menezes LM,Dental»
Neves BM, Motta BB, Azeredo F, Quintão CCA -Life cycle phases: Literature review and new classification proposal for application in healthcare Phase characterized by mixed dentition; starts with the eruption of the first permanent molars.2

Figure 12 :classification proposal for application in healthcare 29 Dental
Figure 12: Phases of growth observed on the carpal radiograph illustrated in the growth curve (PGS = pubertal growth spurt, PGSSP = pubertal growth spurt speed peak) (Based on Mercadante 22 2001).

Menezes 33 Dental»
LM, Neves BM, Motta BB, Azeredo F, Quintão CCA -Life cycle phases: Literature review and new classification proposal for application in healthcare Greater susceptibility to tooth fractures and loss; use of dental prostheses and/or implants.

Menezes LM, Neves BM, Motta BB, Azeredo F, Quintão CCA -Life cycle phases: Literature review and new classification proposal for application in healthcare 25 Dental
described the life cycle in six stages.The classification by Havighurst 13 has 6 phases and is based on socio-cultural, learning, and function factors.However, it does not highlight the issue of the development of persons, which would be a use limitation for areas that involve growth and development.Gallahue et al. 15 proposed a life cycle classification with well-established age groups, phases, and subdivisions, with chronological age as a criterion for definition.Press J Orthod.2023;28(5):e23spe5

Table 1 :
Relationship between the phases observed on the carpal radiography and the growth characteristics (Based onMercadante 22 2001).

Table 2 :
Phases of the proposed life cycle (The 10-phase Life Cycle), with the striking and frequent characteristics of each phase.